Process for recovering a desired soluble salt from a solid mixture containing same



Oct. 7, 1952 L. D. ANDERSON 2,613,133

PROCESS FOR RECCVERING A DESIRED SOLUBLE SALT FROM A SOLID MIXTURECONTAINING SAME Filed Jan. 18, 1950 A Brine 2 Head Tank Fresh Water Wash4 2 3 Concentrates fifg'f'gf:

. Gri ndi n g Ore Feed and Flota ti on Fresh Waier Tailings 5 .5Ta|||ngs i to Dump :1:

, FILTRATE 8 Evaporated Water to Atmosphere Subsurface Combustion BrineEvaporator INVENTOR. Leslie D. Anderson ATTORNEY Patented Oct. 7, 1952PROCESS FOR RECOVERING A DESIRED SOLUBLE SALT FROM A SOLID MIX- TURECONTAINING SAME Leslie D. Anderson, Carlsbad, N. Mex., assignor toPotash Company of America, a corporation of Colorado Application January18, 1950, Serial No. 139,170

7 Claims. 1

This invention, which is a continuation-inpart of my precedingapplication, Serial No. 559,821, filed October 21, 1944, and nowabandoned, the disclosure of which is hereby incorporated herein byreference, relates to processes for treating ores and similar mixturesof water-soluble salts to separate and purify one or more of the salts,and particularly to such processes in which a saturated solution of thesalts is circulated in a closed circuit.

In the various processes for treating ores or other mixtures to separatewater-soluble salts therefrom and to purify the salts, masses of thesalts are solidified from the solution under treatment, or one salt isseparated from another in the solution which acts as a carrier vehicle,and it is necessary to filter the masses to remove the solution; it isalso desirable to wash the masses with water to remove excess solution,and particularly to remove one constituent in solution from anothersolids constituent of the treatment.

In such processes in which a saturated solution is continuouslycirculated through the several stages of the process and is returned'tothe initial stage, the amount of water which can be added to thecirculating solution is limited in accordance with the quantities ofwater which are removedwith the solid products and wastes. 'Ifhiswashing which maintains the balance of the process isknown as a onehundred per cent wash. Greater purity of the product canbe attained bymore complete washing, and a one hundred and fifty to two hundred percent wash is desirable; however, the addition of the excess water to thecirculating solution upsets the balance of the cyclic process; on theother hand, if theexcess washwater is discarded with the wasteyanappreciable amount of the salt which is dissolved inthe wash water islost.

Accordingly, it is an object of this invention toprovide in a cyclicprocess for treating ores to separate purified salts and involvingthecontinuous' circulation of a saturated solution of the salts, animproved method for insuring complete washing of the solidified saltmass without upsetting the balance of the process cycle. N It is anotherobject of this invention to provide an improved cyclic process fortreating ores containing water-soluble salts to separate and purify amass of salt and to wash the separated mass with an excess of waterwhile maintaining the balance of the process cycle and without wastingthe salts dissolved in the wash water.

It is an additional important object of myinvention to provide a processfor thermally treating solutions of water-soluble salts, particularlythe continuously circulating brine utilized as a pulp carrier inrefining operations for an ore containinga plurality of water-solublesalts, whereby improved separation of the soluble components isattained.

Further objects and advantages of this invention will become apparentfrom the following description and the features of novelty whichcharacterize the invention will be pointed out with particularity in theclaims annexed hereto and forming a part of this specification.

For a better understanding of the invention, reference may be had to theaccompanying drawing, the single figure of which is a flow sheet ordiagrammatic illustration of a mill arranged for operation in accordancewith the process of the invention to separate purified potassiumchloride from ores such as sylvinite ore.

. sodium chlorides is introduced into a saturated solution of thechlorides and sodium chloride is removed by flotation, the residuecomprising solidified potassium chloride forming a mass which is thenfiltered and washed to provide substantially pure potassium chloride.The sodium chloride and impurities of the ore removed by flotation areconducted as a pulp in a saturated brine to a thickener, the thickenedpulp product of which is subjected to filtration and washing to removethe solution which still contains potassium chloride. The particularflotation treatment is not a part of the present invention and adetailed description is not necessary to an understanding of the presentinvention.

Briefly stated, the operation involves a grinding stage in which thesoluble constituents are unlocked. This operation preferably involveswet grinding in closed circuit with a classifier with a saturatedsolution of the soluble constituents acting as the liquid phase. Theoverflow of the classifier passes to the flotation stage where theaforesaid separation is attained,

In ore treatment processes such as that abovedescribed, the amount ofwater which may be used for washing the concentrates and the wasteproducts or tailings is limited by the amount of water which is removedfrom the cycle with those solid materials. In the process of thisinvention an excess of water is employed for washing the concentratesand tailings filter cakes, and the wash water filtrate is returned tothe circulating solution. A portion of the solution is thereafterWithdrawn and separately heated to vaporize the water and drive off theexcess, resulting in the formation of a sludge of sodium chloride, Whilemaintaining any potassium chloride in solution. The sludge and remainingsolution in the withdrawn portion are then returned to the treatmentcycle at the thickener stage. The quantity of water required to balancethe process is thus maintained while aifording complete washing of theconcentrates and tailings and recovery of the valuable constituentsremoved with the wash water.

The temperature and control attained in the evaporation process isimportant and applicable to the treatment of solutions containing two ormore water-soluble salts regardless of whether or not flotationprocedures are utilized in attaining the primary or principalseparation. For example, in the system described, the solubility ofpotassium chloride in water becomes greater with an increase intemperature than does the solubility of sodium chloride. If, therefore;a brine saturated with respect to sodium and potassium chlorides is tobe refined, the solution may be heated to a second temperature higherthan its initial temperature and suificient water evaporated toapproximately concentrate the heated solution with respect to potassiumchloride. Under such circumstances the sodium chloride having exceededits solubility at the second temperature will precipitate and may bedischarged or otherwise treated, while the concentrated solution nowenriched with respect to potassium chloride is recycled into the systemfor further use. If desired, additional water can be added to thissolution, thereby adjusting the concentration thereof and maintainingthe desired liquid balance.

In utilizing my invention, I prefer to operate the evaporator in suchmanner that both effects are attained; namely, that a predeterminedquantity of water is removed from the system to compensate for theadditional water in the wash operation, and that when such quantity ofwater has been removed the solution will be saturated with respect topotassium salts, but will have exceeded saturation with respect to thesodium salts, thereby occasioning precipitation of such salts. Insystems where the reverse of the above-described situation exists, thedesired salts may be precipitated during the evaporation operation.

Under such circumstances it may obviously be collected and subsequentlyrefined as desired.

Referring now to the flow sheet shown on the drawing, a brine comprisinga saturated solution of sodium chloride and potassium chloride in wateris supplied from a plant stock tank 2 to a grinding and flotationcircuit 3 and at the same time the ore such as sylvinite is fed to thegrinding stage. The ore is finely ground and introduced into thesaturated solution within the circuit 3, preferably by Wet grinding in aball mill of said circuit. Sodium chloride and gangue matter are removedby fiotation leaving a residue or pulp containing substantially puresolidified potassium chloride. This concentrated residue is thensupplied to a filter 4, which may be a centrifuge or other suitablefiltering apparatus, Where the solution is filtered from the solidifiedmass of potassium chloride. The mass or cake of filtered concentrate isthen conveyed to drying apparatus where excess moisture is removed,leaving the final product.

The froth removed during the flotation stage and which contains sodiumchloride and gangue matter together with some saturated solution of thechlorides is conducted from the apparatus to a thickening apparatus 5,and the thickened pulp carrying the solid sodium chloride is thenconducted to a filter or centrifuge '5, where the solid sodium chlorideand solution are separated.

The filtrates from both the filters 4 and 6 are conducted back to thestock tank 2 through a suitable return conduit or path I. It is assumedhere that the purpose of the plant represented on the flow sheet is toseparate pure potassium chloride concentrate and that other solid matterincluding sodium chloride is discharged to the dump as tailings. It isobvious that some of the solution which wets the masses or cakes offiltered concentrate and tailings may be removed from the filters A and6 with the solid matter and that some of the solution may be lost to theclosed circuit. It is therefore desirable to wash the solidified masseswith pure water, however if more water is addedthan is to be removedwith the solids, then the balance of the cycle is upset. Furthermore,the filtrates and Wash Water contain all the constituents of thesolution including potassium chloride and any necessary reagentsintroduced for effecting the flotation treatment.

In order to employ excess water Washing of the separated masses ofconcentrates and tailings, a portion of the returning solution in theconduit 1, which contains the excess wash water employed in the filters,is removed and supplied to an evaporator 8. Here water is vaporized toremove the excess Water and a sludge of sodium chloride is formed; andthis sludge, which includes some of the solution, is then reintroducedin the cycle at the thickener 5. By the operation of the evaporator 8,the excess water is continuously removed from the cycle and the correctbalance is maintained. The remaining portion of the solution in theconduit l is returned to the stock tank 2.

The evaporator 3 may be of any suitable construction which affordscontrollable heating of the solution; for efiicient operation anevaporator of the sub-surface combustion type has been found preferable.Evaporators of this type are provided with an arrangement for forcing acombustible mixture of gases under suitable pressure into a burner belowthe surface of the solution. Combustion takes place in the burner andthe hot gases pass into the solution. The gases together withvaporizedwater are then exhausted to the atmosphere. Conditions insub-surface combustion evaporators are such that evaporation takes placeat temperatures below the normal boiling point of the solution. Thischaracteristic is desirable in the present process to maintain lowertemperatures of the hot sludge resulting from the evaporating processand which is supplied to the thickening stage. Furthermore, thetemperature of the evaporating operation should be controlled withrespect to the. relative solubilities of the salts in the solution toprevent solidifying any of the potassium chloride while forming a sludgecontaining solid sodium chloride. This regulation of temperature bymaintaining the potassium chloride in solution prevents the loss ofpotassium chloride to the dump with the solidified sodium chloridetailings.

It will be understood readily by those skilled in the art that theevaporating method of this invention may be applied to the processherein entire liquid balance. v process is applicable to other refiningsystems cult and in which excess fresh water washing of the finalproduct is desirable in order to obtainpure'r concentrates and to entaillesswaste "of the solution. As previously noted, it is not essentialthat the evaporation step of my procedure be utilized in connection withthesystem' incorporating the froth flotation. 'On the contrary, brinescontaining a desired soluble salt and one 01' more additionalundesirable salts may be concentrated by evaporation' as ,hereindescribed, provided the solubility of one 'offthe salt componentsincreasesmore rapidly with the increase of temperature than does thesolubility of another of the salt components. If the systemdoes notcontain excess water, the

quantity of water evaporated in attaining c'oncentration of one of thesalt components at an elevated temperature may beconden'sed and'returned to the system, or an equivalent amount of water subsequentlyadded to maintain the Thus this portion of my of ore treatment. Forexample, this process may be employed in; the flotation separation ofpotash, borax, sodium sulphate and potassium nitrate from theirrespective ores, as well asin similar operations treating chemical saltsof whatever origin.

Accordingly it is not desired that the invention be limited to theparticular application of the process described and illustrated, and itis intended --by the accompanying claims to cover all modificationswithin the spirit and scope of the invention.-

1. A process for separating a desired component salt from admixture in awater-soluble ore, which comprises the steps of mixingthe finely dividedore with a solution saturated with .the component salts comprising saidore, removing an undesired component salt from said ore by, flotationwith the desired salt formed as a concentrate, separating the saidother'salt from solution entrained therewith after its removal from theflotation stage, separating the desired salt from the saturated solutionin the concentrate, washing at least one of the component salts soseparated with water and combining said wash water with the solutionpreviously separated from at least one of the salts, evaporating aportion of the liquid mixture so formed to a concentration such that ata determined temperature of evaporation an amount of water is removedwhich is substantially equivalent to the amount of water used in saidwashing operation, thereby forming a salt sludge while preventingcrystallization and precipitation of the desired component salt fromsolution, returning said sludge to one of said prior separation stagesto separate associated solution from its solids content, and recyclingthe solution remaining after said evaporation for mixing with the oreprior to flotation, so that any crystallization of the desired componentsalt will occur not later than discharge from the flotation stages andwill thus accompany the concentrates.

2. A process for separating a desired component salt from admixture withan undesired salt in a water-soluble ore, which comprises the steps ofmixing the finely divided ore with a solution saturated with thecomponent salts comprising said ore, removing the undesired componentsalt from said ore by flotation with the desired salt formed as aconcentrate, separating the said other salt from solutionentrainedtherewith after its removal from the flotation stage, separating thedesired salt from the saturated solution in the concentrate, washing thecomponent salts so separated with water and combining said wash waterwith the solution previously separated from the salts, evaporating aportion of the liquid mixture so formed to a concentration such that ata determined temperature of evaporation an amount of water is removedwhich is substantially equivalent to the excess of water used in saidwashing operation, thereby maintaining the liquid balance of the processand forming a salt sludge while preventing crystallization andprecipitation of the desired component salt, returning said sludge toone of said prior separation stages to separate associated solution fromits solid content, and recycling the solution remaining after saidevaporation for mixing with the ore prior to flotation so that anycrystallization of the desired component salt will occur not later-thandischarge from the flotation stage and will thus accompany theconcentrates.

3. 'In a cyclic process for recovering a desired soluble salt componentfrom a mixture of solids with which the component is associated, thesteps of introducing said mixture into a solution saturated with respectto said desired soluble salt,

separating the desired soluble salt from iother solids contained inthesolution, subjecting the soluble salt composition so separated toafilter ing treatment, washing the filtered solids'with water, filteringthe other solids, combining said wash water with the filtrates of saidfiltering operations, evaporating a portion of the combined filtratesand wash water to a concentration such that a determined temperature ofevaporation an amount of water is removed which is at leastsubstantially equal to the excess of water used to wash the filteredsolids, and returning the combined filtrate and wash water solution soadjusted to saturation as the saturated solution into which the solidsmixture is introduced, whereby the liquid balance of the process ismaintained.

4. In a cyclic process for recovering a desired soluble salt componentfrom a mixture of solids with which the component is associated, thesteps of introducing said mixture into a solution saturated with respectto said desired soluble salt, separating the desired soluble salt fromother solids contained in the solution, subjecting the soluble saltcomposition so separated to a filtering treatment, separating the saidother solids from the solution, washing the filtered solids with water,mixing said wash water and solution with the filtrates of said filteringoperation, evaporating a portion of the mixture to a concentration suchthat at a determined temperature of evaporation an amount of water isremoved which is substantially equivalent to the excess of water used towash the filtered solids by introducing a combustible mixture of gasesunder pressure below the surface of said combined filtrates and washwater and burning said gases while maintained in the sub-surfaceposition, and returning the resultant mixture as a solution so adjustedto saturation as the saturated solution into which the solids mixture isintroduced, whereby the liquid volume in the process remainssubstantially constant.

5. A process for separating purified KCl from sylvinite which comprisesthe steps of mixing finely divided ore in a saturated solution of sodiumand potassium chlorides, separating NaCl from the mixture by flotation,filtering solid KCI from the saturated solution in the K01 concentrates,thickening the froth floated NaCl, filtering NaCl from the NaCl sludgefrom the thickening operation, washing the filtered KCl and NaCl withwater, combining said wash waters with said filtrates, evaporating aportion of the combined filtrates and wash water to a concentration suchthat at a determined temperature of evaporation an amount of waterisremoved which is equivalent to the amount of water used to wash thesolid KCl and NaCland a NaCl sludge is formed while maintaining the KClin solution, returning said sludge to the thickening operation,returning the remaining portion of the combined filtrates and washwaters adjusted to saturation as the saturated solution to mix with thefinely divided ore.

6. In a process for recovering a desired soluble salt component from amixture of solids containing the desired soluble salt component and anundesired soluble salt component and in which the increase in solubilityattained by an increase in temperature is greater for the desired saltthan for the undesired salt, the steps of introducing said mixture ofsolids into a solution saturated with respect to said desired solublesalt, separating the desired soluble salt from the other solidscontained in the solution, subjecting the separated solids to afiltering treatment, washing the filtered solids with water, combiningthe filtrate and wash water of said filtering operations, heating atleast a portion of the combined filtrates and wash Water to a secondtemperature to induce evaporation, continuing said evaporation untilsaid portion of the solution is approximately saturated with respect tothe desired soluble salt at said second temperature, discharging theprecipitated solids from the solution, and recycling the concentratedportion of the solution for mixture with the untreated portion forsubsequent mixture with said mixture of solids.

'7. The cyclic process for separating a first salt from admixture with asecond salt in a watersoluble ore, which comprise the steps of mixingground ore with a circulating aqueous solution saturated with respect tothe said firstand second salts, separating the first salt from thesecond salt by froth flotation, separating the salt from solutionentrained therewith after its removal from froth flotation stage,separating the second salt from the solution entrained therewith afteritsremoval from the froth flotation stage, washing at least one of thesalts with water, combining the Wash water with the circulatingsolution, evaporating from a portion of the resulting solution aquantity of water sufiicient to maintain the quantity of circulatingsolution at a substantially constant volume, said evaporation beingconducted at a determined temperature for forming a sludge of the firstsalt while preventing crystallization of the second salt, returning thesaid sludge to one of the prior separation stages for separatingentrained solution from the solids, and mixing the solution remainingafter evaporation with the circulating solution prior to the flotationstage so that any crystallization of the second salt will occur beforedischarge from the flotation stage and the second salt particles socrystallized will accompany the concentrates.

LESLIE D. ANDERSON.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS I OTHER REFERENCES fSolubilities of Inorganic andOrganic Compounds," by A. Seidell, pp. 641 and 643, 2nd ed., 2ndprinting, D. Van Nostrand Co., Inc., N. Y.

1. A PROCESS FOR SEPARATING A DESIRED COMPONENT SALT FROM ADMIXTURE IN AWATER-SOLUBLE ORE, WHICH COMPRISES THE STEPS OF MIXING THE FINELYDIVIDED ORE WITH A SOLUTION SATURATED WITH THE COMPONENT SALTSCOMPRISING SAID ORE, REMOVING AN UNDESIRED COMPONENT SALT FROM SAID OREBY FLOTATION WITH THE DESIRED SALT FORMED AS A CONCENTRATE, SEPARATINGTHE SAID OTHER SALT FROM SOLUTIN ENTRAINED THEREWITH AFTER ITS REMOVALFROM THE FLOTATION STAGE, SEPARATING THE DESIRED SALT FROM THE SATURATEDSOLUTION IN THE CONCENTRATE, WASHING AT LEAST ONE OF THE COMPONENT SALTSSO SEPARATED WITH WATER AND COMBINING SAID WASH WATER WITH THE SOLUTIONPREVIOUSLY SEPARATED FROM AT LEAST ONE OF THE SALTS, EVAPORATING APORTION OF THE LIQUID MIXTURE SO FORMED TO A CONCENTRATION SUCH THAT ATA DETERMINED TEMPERATURE OF EVAPORATION AN AMOUNT OF WATER IS REMOVEDWHICH IS SUBSTANTIALLY EQUIVALENT TO THE AMOUNT OF WATER USED IN SAIDWASHING OPERATION, THEREBY FORMING A SALT SLUDGE WHILE PREVENTINGCRYSTALLIZATION AND PRECIPITATION OF THE DESIRED COMPONENT SALT FROMSOLUTION, RETURNING SAID SLUDGE TO ONE OF SAID PRIOR SEPARATION STAGESTO SEPARATE ASSOCIATED SOLUTION FROM ITS SOLIDS CONTENT, AND RECYCLINGTHE SOLUTION REMAINING AFTER SAID EVAPORATION FOR MIXING THE ORE PRIORTO FLOTATION, SO THAT ANY CRYSTALLIZATION OF THE DESIRED COMPONENT SALTWILL OCCUR NOT LATER THAN DISCHARGE FROM THE FLOTATION STAGES AND WILLTHUS ACCOMPANY THE CONCENTRATES.